Method of junction formation for CIGS photovoltaic devices

ABSTRACT

Sulfur is used to improve the performance of CIGS devices prepared by the evaporation of a single source ZIS type compound to form a buffer layer on the CIGS. The sulfur may be evaporated, or contained in the ZIS type material, or both. Vacuum evaporation apparatus of many types useful in the practice of the invention are known in the art. Other methods of delivery, such as sputtering, or application of a thiourea solution, may be substituted for evaporation.

PRIORITY

This application claims priority from Provisional Application Ser. No.60/331,867, filed Nov. 20, 2001.

GOVERNMENT SPONSORED RESEARCH

This invention was made with government support under NREL SubcontractNo. ZAK-8-17619-21, Prime Contract No. DE-AC36-99GO10337 awarded by theDepartment of Energy. The government has certain rights in thisinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is in the field of photovoltaic devices, such as solarcells.

2. Brief Description of the Background Art

Chalcopyrite semiconductors based on CuInSe₂ (or CIS) have beeninvestigated for their application to thin film solar cells for over 25years. Chalcopyrite semiconductors are formed from group I, group III,and group VI elements of the periodic table. Alloying of CuInSe₂ andCuGaSe₂ to form Cu(In_(x)Ga_(1-x))Se₂ (or CIGS) allows the energy gap tobe varied between 1.0 eV and 1.68 eV in order to vary the spectralabsorption profile. Alloying with CuInS₂ to formCu(In_(x)Ga_(1-x))(Se_(y)S_(1-y))₂ (or CIGSS) allows band-gaps as highas 2.4 eV to be obtained. These semiconductors are direct gap materials;their high optical absorption coefficient allows absorption of sunlightin layers that are only 2 μm in thickness. The electrical properties ofthe material are determined by composition, intrinsic defects, andstructural defects. The cost of solar energy conversion has already beenlowered through use of thin film amorphous silicon photovoltaic modules,and a further cost reduction may be anticipated via the use of highefficiency thin film CIGS devices.

Although CIS materials generally deviate from the exact stoichiometry ofCuInSe₂, it is found that they are usually described by thepseudo-binary system (1−δ)Cu₂Se+(1+δ)In₂Se₃. Electronically useful filmsare Cu-poor (Cu/In<1, or more generally, Cu/(In+Ga)<1), and are p-type.All efficient solar cells are made using such p-type material. InCu-rich films (Cu/(In+Ga)>1) the degenerate semiconductor Cu_(x)Seforms, leading to films of a metallic nature. Solar cells made usingCu-rich films are of very poor quality.

A solar cell is essentially a rectifying junction in a semiconductormaterial in which light can be absorbed. The free electrons and holesgenerated by the absorption of photons are separated by an internalelectric field in the semiconductor, giving rise to a photovoltage. Inprinciple, two types of junction can be envisaged using p-type CIGS. If,for example, a surface layer of the CIGS is made n-type throughintroduction of n-type dopant atoms, then an n-p homojunction is formed.Alternatively, band-bending in the CIGS can be induced by deposition ofan n-type semiconductor material of a completely different composition,thereby forming a heterojunction.

The first solar cells made using CIS as the semiconductor employed alayer of n-type CdS deposited by vacuum evaporation to form what wasthought to be a heterojunction. Later, to allow the thickness of the CdSto be reduced, the CdS was overcoated with a transparent conductor(Al-doped ZnO). It was also discovered that the use of CdS layersprepared by chemical bath deposition (CBD) using, for example, anaqueous ammonium hydroxide solution containing cadmium acetate as a Cdsource and thiourea as a sulfur source, allowed solar cells of higherconversion efficiency to be produced.

The full structure of this type of thin film cell isZnO/CdS/CIGS/Mo/glass, where the Mo serves as an ohmic contact at therear of the device. Usually, the ZnO is deposited as a bi-layerconsisting of about 500 A (Angstroms) of high resistivity ZnO (i-ZnO)followed by about 4000 A of highly conductive ZnO (ZnO:Al). The CdS(together with the i-ZnO) is frequently referred to as a buffer layerthat is inserted between the active CIGS layer and the ZnO transparentconductor. The use of a Na-containing glass, e.g. soda-lime glass, isanother factor that contributes to the achievement of high efficiencies.In 1999, a record 18.8% total-area conversion efficiency was reportedfor a CIGS solar cell with this structure (M. A. Contreras, B. Egaas, K.Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, and R. Noufi, Prog.Photovolt: Res. Appl. 7, 311–316 (1999)). The CIGS was deposited by athree-stage process based on vacuum evaporation (see U.S. Pat. Nos.5,441,897 and 5,436,204).

While the use of CBD CdS for junction formation has resulted in thehighest conversion efficiencies in the laboratory, its use in highvolume manufacturing is problematic owing to the presence of cadmiumboth in the manufacturing plant and in the product. The generation oflarge volumes of liquid chemical waste is also a nuisance and asignificant cost factor. Consequently, the elimination of the wet CBDCdS step in producing CIGS solar devices and its replacement by a dryprocess represents an important practical goal.

The attainment of this goal has been sought by researchers around theworld and has proven elusive. Omission of the buffer layer usuallyresults in solar cells of very poor efficiency. A study of theliterature reveals that over twenty other materials and ten depositionprocesses have been investigated in the hope of forming satisfactorybuffer layers. Materials include ZnS, ZnSe, ZnO, ZnIn_(x)Se_(y),Zn(O,S,OH)_(x), In(OH,S)_(x), In₂Se₃, CdSe, CdCl₂, Sn(S,O)₂, Zn₂SnO₄,and a-Si:H. Methods include CBD, evaporation, co-evaporation,sputtering, MOVPE (Metal Organic Vapor Phase Epitaxy), MOCVD (MetalOrganic Chemical Vapor Deposition), ALE (Atomic Layer Epitaxy), solvent,and PECVD (Plasma Enhanced Chemical Vapor Deposition). No dry processhas yet equaled the combination of high cell efficiency and high deviceyield achievable with CBD CdS.

It appears that CBD CdS confers multiple and distinct benefits. Thesemay well include:

-   -   cleaning of the CIGS surface, possibly involving removal of        native oxide, and dissolution of sodium carbonate    -   complete physical coverage of the CIGS    -   provision of a buffer layer of high, but finite, resistivity,        thereby lessening the effect of electrical shunts    -   in-diffusion of Cd (possibly by Cu—Cd ion exchange) and n-type        doping of the surface by Cd donors    -   provision of a barrier to sputter damage of the CIGS during ZnO        deposition    -   removal of interface acceptors, possibly arising from oxygen on        Se sites    -   improvement of the minority carrier diffusion length in the film        bulk        The fact that CBD CdS produces better devices than vacuum        evaporated CdS demonstrates that the performance of the buffer        layer depends not merely on its chemical composition but on the        method of deposition.

It has been reported that Cd and Zn are n-type dopants in CIS. Since thegrowth of CBD CdS takes place at the low temperature of about 60–80° C.,making it unlikely that true epitaxial growth of CdS on CIS takes place,we assume that the highly efficient CIGS devices are, in fact,homojunctions in which the junction is buried at a small depth insidethe CIGS, thereby negating the need for epitaxial growth to ensure a lowdefect density at the junction interface. We further hypothesize thatduring CBD, Cd diffuses a short distance into the CIS.

SUMMARY OF THE INVENTION

In view of the above, the inventor established a line of work based on aZn-containing buffer layer to offer the possibility of forming a shallown-p homojunction in the CIGS absorber. Furthermore, since In and Se arealready present in CIS or CIGS, and therefore cannot be regarded asimpurities, the use of ZnIn_(x)Se_(y) (ZIS) as a buffer material wasinvestigated. (Note that in the abbreviation ZIS, the S stands forselenium.)

This material (ZIS) had been applied to CIGS as a buffer material byKonagai et al. (M. Konagai, Y. Ohtake, and T. Okamoto, MRS Symp. Proc.Vol. 426 (1996) pp 153–163). In this work, the ZIS was formed directlyon the CIGS surface by three-source evaporation, i.e., by simultaneousevaporation of the elements Zn, In, and Se from three separate sources.

In distinction to this work, we proposed to chemically synthesize thebulk compound ZnIn₂Se₄ (and related compounds), and then to use thismaterial as a single evaporation source material to form a ZIS typebuffer layer. Advantages of this method include simplification of thedeposition hardware, simpler deposition control, and a fixed filmcomposition.

This proposal was carried out and working CIGS devices were successfullyprepared using evaporation of ZnIn₂Se₄ onto CIGS to form a ZIS bufferlayer. The work was reported at the 16^(th) European Photovoltaic SolarEnergy Conference, May 2000, in Glasgow (A. Delahoy, M. Akhtar, J.Bruns, A. Ruppert, L. Chen, Z. Kiss, Conf. Proc. pp 767–770). ZnIn₂Se₄is a representative of a class of compounds known as defect chalcopyritesemiconductors. The defect chalcopyrites are formed from group II, groupIII, and group VI elements of the periodic table.

The subject of this invention concerns the use of sulfur in improvingthe performance of CIGS devices prepared by the evaporation of a singlesource ZIS type compound to form a buffer layer on the CIGS. The sulfurmay be evaporated, or contained in the ZIS type material, or both.Vacuum evaporation apparatus of many types useful in the practice of theinvention are known in the art. Other methods of delivery may besubstituted for evaporation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of deposition rate as a function of evaporation timeand source temperature.

FIG. 2 is a graph of a current-voltage curve of a ZIS/CIGS solar cellwith an efficiency of 11.5%

FIG. 3 is a graph of quantum efficiency (QE) curves for a ZISS/CIGSsolar cell exhibiting a QE of 0.7 at 400 nm, and for a matched pair ofZIS/CIGS cells fabricated with and without sulfur.

FIG. 4 is a schematic representation of an exemplary vacuum evaporationapparatus.

DETAILED DESCRIPTION OF THE INVENTION

Synthesis of ZnIn₂Se₄

ZnIn₂Se₄ was synthesized in two stages: first, the preparation of aZnIn₂ alloy, and second, the reaction of the alloy with Se. The weightratio of the constituent elements in stoichiometric ZnIn₂Se₄ is1:3.51:4.83, as calculated from the atomic weights of Zn, In, and Se,respectively.

The source materials were 1–5 mm pieces of 99.99% Zn, and In shot99.999%. 5 g of Zn was used, and 17.55 g of In. The In was cut intopieces of roughly equal size to the Zn, and the Zn and In were placedinto a vertical quartz reactor. The reactor was purged with N₂ and washeated under N₂ flow to 320° C. The mixture was fully liquid. The heaterwas turned off while stirring. The alloy appeared to solidify at 170° C.The Zn:In alloy formed a sticky layer at the bottom of the quartzreactor and was removed as a solid circular plate.

The edge of the Zn:In plate was cut into small pieces. Thestoichiometric amount of Se was calculated to be 24.15 g. A further 3 gwas added. The total starting weight was therefore 46.7 g+3 g. The Sewas ground well, and mixed with the Zn:In alloy pieces. The Zn:In and Semixture was placed at the bottom of a glass tube liner, with theremaining large piece of Zn:In alloy on top of the mixture. The linerwas placed in a stainless steel reactor. The reactor was purged with N₂and heated under N₂ purge. At 100° C. the reactor was sealed. Thereactor was heated to 460° C. and then cooled. Most of the product wasfound as a black, brittle material at the bottom of the tube, with asmaller amount on the walls. The total product of ZnIn₂Se₄ was 46 g.

Other ZIS type materials were synthesized using different ratios of Znand In, i.e., ZnIn_(x)Se_(y). Some of these could be described asZnSe:In or In₂Se₃:Zn.

Synthesis of ZnIn₂ (Se_(1-x), S_(x))₄

A ZIS type source material was also prepared in which sulfur wassubstituted for some of the selenium atoms. We shall abbreviate thismaterial as ZISS.

Synthesis of ZnGa₂Se₄ (We shall abbreviate this material as ZGS.)

In this case, the Zn, Ga, and Se were mixed and heated to 470° C. Theproduct was ground, mixed will and heated again to 470° C. to form thefinal product.

Evaporation of ZIS-Type Materials

Evaporation of the ZIS was conducted after weighing the desired amountand placing it in an open tungsten boat in the vacuum system. The boatwas heated electrically until the complete charge of ZIS had beenevaporated. Later, a small graphite crucible heated by an externaltungsten foil heater was used with equal success. For solar cells, amass of ZIS in the range 10 to 20 mg proved satisfactory. Smaller massesresulted in thinner films with greater transparency. In experimentsinvolving sulfur, a mass of S in the range 10–20 mg was employed, andmost often 20 mg. It was found that evaporation of a 200 mg charge ofZIS (plus 20 mg S) from the graphite crucible resulted in a filmthickness of 2200 Angstroms (2200 A or 220 nm). The thickness wasmeasured by stylus profilometry. From this we infer that the ZIS filmsused in solar cells had a thickness roughly in the range 100 A–220 A (11nm–22 nm). Depending on the geometry of the desired device, the vaporflux can be restricted to approximate a point source or a line source

The deposition rate of material was studied as a function of sourcetemperature during the complete evaporation of a fresh charge ofZnIn₂Se₄. A small emission peak was discovered at a source temperatureof about 290° C., followed by the principal emission peak at about850–860° C., with no other peaks at higher temperatures (see FIG. 1).(Analysis, by in-house EDX, (Energy Dispersive X-ray analysis), ofmaterial captured on a glass slide during the initial emission revealedit to be selenium. In conclusion, it appears that ZnIn₂Se₄ largelyevaporates congruently, and not as different compounds at differenttemperatures.

In most of the experiments using elemental sulfur, the sulfur wasweighed, and both the ZIS and S were placed in the same boat orcrucible. In this case, as the crucible was heated, the sulfur fullyevaporated from the crucible before the ZIS started to evaporate.

Sputtering of ZIS-Type Materials

It will be appreciated by those skilled in the art that the thin film ofZIS (or ZISS) material can also be deposited by sputtering. Magnetronand hollow cathode configurations represent particularly useful types ofsputtering source. Depending on the type of electrical power supplyconnected to the cathode (on which the target of the material to besputtered is mounted), the sputtering mode can be DC, mid-frequencyasymmetric pulsed power, or RF. Furthermore, materials such as ZnIn₂Se₄can be deposited either by sputtering of a compound target containingall three elements, or by reactive sputtering of a metallic target. Inthe latter case, the Se is directed at the substrate either in vaporizedelemental form or in a compound such as H₂Se gas. The metallic targetcould be a Zn—In alloy or it could consist of separate Zn and Insub-targets.

Fabrication of Solar Cells with and without Sulfur

In order to unequivocally test the effect of sulfur, cells within majorhorizontal rows of Table 1 were processed using the same batch of CIGS,the same deposition procedures, and, except for runs 44 and 45, the samedeposition time for sputtered ZnO. (In run 44 the time ZnO time was 10m, while in run 45 it was 7 m). From major row to major row, the batch(and hence quality) of CIGS was different, and parameters such as ZISthickness and substrate temperature for deposition were varied in orderto optimize the process. Whether elemental sulfur is evaporated as wellas the ZIS type material is indicated in the Comment column. In allcases, the S was placed in the same crucible as the ZIS, except in run58 where the sulfur was slowly evaporated in parallel with the ZIS froma second heated source (boat).

TABLE 1 Photovoltaic parameters for solar cells with ZIS or ZISS bufferlayers prepared with and without evaporation of sulfur (referenceperformance with CBD CdS included where available). V_(oc) J_(sc) FFEfficiency Run # Buffer Comment (mV) (mA/cm²) (%) (%) 4 ZIS No S 41725.7 42.6 4.7 5 ZIS S 443 23.9 50.6 5.9 CdS 446 27.5 61.5 7.5 9 ZIS No S432 27.5 55.5 6.6 10 ZIS S 455 31.9 58.6 8.5 CdS 451 30.5 60.9 8.4 17ZIS S 442 31.8 52.6 7.4 CdS 455 35.0 58.5 9.3 22 ZIS S 560 32.1 64.311.5 25 ZGS/ZIS Bi- 465 32.7 53.5 8.1 layer, S 44 ZIS No S 395 24.8 59.65.8 45 ZIS S 426 27.1 55.1 6.4 46 ZIS No S 424 25.5 52.9 5.7 47 ZIS S459 26.7 53.3 6.5 52 ZIS No S 468 29.3 50.8 7.0 51 ZIS S 493 30.4 62.99.4 57 ZIS S 468 26.1 51.7 6.3 58 ZIS S (2^(nd) 464 26.0 54.1 6.5source) CdS 478 31.2 63.1 9.4 59 ZIS S 492 29.2 54.9 7.9 61 ZISS No S485 28.7 54.5 7.6 60 ZISS S 488 30.6 55.2 8.2 CdS 519 34.3 69.0 12.3 63ZIS S 434 29.0 52.8 6.6 64 No buffer No S 263 22.4 32.5 1.9

Analysis of the data in Table 1 shows that the inclusion of sulfur inthe crucible along with the ZIS increases each of the three basic PVparameters, the improvement in average fill factor (FF) and averageopen-circuit voltage (V_(oc)) being about 7%, and the improvement inaverage short-circuit current density (J_(sc)) being about 5%. Sincethese factors are multiplicative in determining solar cell efficiency(efficiency=V_(oc)×J_(sc)×FF), a most useful gain in average efficiencyis achieved (23%) through the use of sulfur and ZIS relative to ZISalone.

Note. The fill factor (FF) is defined as (V_(mp)×I_(mp))/V_(oc)×I_(sc),where V_(mp) is the voltage at the maximum power point of the solarcell's I–V curve, and I_(mp) is the current at the maximum power point.

In run 22 a solar cell efficiency of 11.5% was achieved (see FIG. 2).This run did not have a corresponding result for CBD CdS. In run 25 asolar cell efficiency of 8.1% was achieved by the evaporation of sulfurfollowed by ZGS and then by ZIS. In runs 63 and 64 the overall benefitof the ZIS buffer layer was re-checked. Run 63 (ZIS+S) gave anefficiency of 6.6%, while run 64 (in which the ZIS was omitted) yieldedhighly variable devices of which the best had an efficiency of 1.9%.

Further experiments were conducted in order to investigate the effectsof ZIS deposition temperature, quantity of sulfur, and ZIS thickness.Each series of experiments, as reported in Table 2, was conducted usingpieces of CIGS from the same CIGS run. (Different series used CIGS fromdifferent runs. Consequently, data cannot be directly compared betweenseries because of the different quality and properties of the CIGS.)

TABLE 2 Photovoltaic parameters for solar cells with ZIS buffer layerswith variation of deposition conditions V_(oc) J_(sc) FF Efficiency Run# Buffer Sulfur Temperature (mV) mA/cm² (%) (%) ZIS depositiontemperature series (best device parameters)  73 ZIS (10 mg) 19 mg 175°C. 481 28.9 65.3 9.1  74 ZIS (10 mg) 19 mg 185° C. 489 29.0 64.6 9.2  75ZIS (10 mg) 19 mg 197° C. 504 29.7 67.3 10.1   76 ZIS (10 mg) 19 mg 210°C. 496 28.4 66.6 9.4 Sulfur series (average device parameters)  79 ZIS(10 mg)  0 mg 197° C. 454 27.1 51.0 6.3  80 ZIS (10 mg)  5 mg 197° C.474 28.5 59.0 8.0  81 ZIS (10 mg) 19 mg 197° C. 463 26.2 56.5 6.8  82ZIS (10 mg) 30 mg 197° C. 472 25.1 58.0 6.9 ZIS thickness series (bestdevice parameters)  87 ZIS (7 mg)   5 mg 197° C. 336 27.0 37.8 3.4  88ZIS (10 mg)  5 mg 197° C. 458 29.9 57.8 7.9  89 ZIS (14 mg)  5 mg 197°C. 397 29.3 53.9 6.3 Control CBD CdS — — 477 33.2 57.1 9.0 Additionalsulfur treatment of CIGS using thiourea (best device parameters) 140 ZIS(15 mg)  5 mg 197° C. 481 30.2 58.4 8.5 thiourea 181A ZIS (15 mg)  5 mg197° C. 409 28.6 44.4 5.2 water 181B ZIS (15 mg)  5 mg 197° C. 430 30.553.3 7.0 thiourea

The temperature series of experiments suggests substrate temperatures inthe range 175–210° C. are usable, with an optimum approximately 197° C.

The sulfur series confirms the utility of sulfur in improvingefficiency, but suggests that an optimum may exist. The optimum sulfurcharge to be loaded into the crucible would seem to be about half themass of the ZIS charge. In general, the mass of sulfur evaporated shouldbe from 0.3 to twice that of the ZIS, but preferably about half that ofthe ZIS. The upper end of the range is approximately four times the massof ZIS or ZISS.

The ZIS thickness series shows that efficiency falls off strongly forthe smallest 7 mg charge. This charge corresponds to a thickness of 77A. The optimum charge corresponds to a thickness of about 110 A,although reasonable results were also obtained at a ZIS thickness of 154A. The 7.9% cell obtained in run 88 had an efficiency of 88% of acontrol cell using CdS.

It would not be unreasonable to assume that further development of cellsincorporating a ZIS buffer layer would enable efficiencies equal tothose with CBD CdS to be obtained.

We note that the bandgap of CdS is 2.5 eV, while that of ZIS is only 2.0eV. These bandgaps correspond to absorption edges near 520 nm and 620nm, respectively. The apparent disadvantage of ZIS relative to CdS dueto absorption in the wavelength range 520–620 nm can fortunately becompensated through the use of thinner layers of ZIS, so that overallabsorption remains comparable. This is demonstrated in the quantumefficiency (QE) data of FIG. 3, in which a QE of 0.7 is achieved at awavelength of 400 nm. The buffer layer for this cell was ZISS evaporatedwith sulfur.

Detailed analysis of ZIS films co-deposited on a witness glass revealedthat many films suffered from sub-gap absorption. It was found thatsubstrate temperature influenced the strength of this absorption. Italso appeared that the use of sulfur reduced this absorption, therebyincreasing the optical transmission of the film. This particular effectcould at least partially account for the increased current densityobtained through the use of sulfur. FIG. 3 reveals the increasedtransparency of ZIS+S relative to ZIS alone, as evidenced by theenhanced QE for short wavelengths. It is notable that the efficiencyobtained with ZISS+S (run 60) slightly exceeded that obtained with ZIS+S(run 59). This demonstrates that ZISS is also a useful material forbuffer layer deposition. The ZISS source composition was ZnIn₂Se₃ ₆S₀ ₄.Pretreatment of the ZISS source material at a temperature significantlylower than the evaporation temperature for a time sufficient to driveoff superficial Se, can be beneficial. The pretreatment temperature ispreferably from 250° C. to 600° C.

Wherever cell results on a particular batch of CIGS are availablepertaining to a standard buffer using CBD CdS, these have been includedin Table 1. Further analysis of the data in Table 1 shows that the useof ZIS (or ZISS) with sulfur on average resulted in a solar cellefficiency equal to 78% of that obtained with CBD CdS. The deficienciesappear to be mostly in fill factor and short-circuit current. However,it is fully possible that further experimentation will close theefficiency gap between evaporated ZIS and CBD CdS. Buffer layersconsisting essentially of ZnIn₂ (Se_(1-x), S_(x))₂, with x less thanapproximately 0.6 and particularly advantageous. The buffer layers arepreferable from 4 nm to 30 nm thick.

Although identification of the mechanism by which the evaporation ofsulfur prior to the deposition of ZIS improves the solar cell is notneeded to support this invention, nevertheless it is believed that atleast two processes occur. One of these is reaction of the S with theCIGS (possibly passivating defect states), and the second ismodification of the ZIS. Because of the high vapor pressure of S atrelatively low temperatures, it is likely that some sulfur impinges onthe growing ZIS film (i.e. after completion of the sulfur evaporation)through its re-release from hot surfaces in the deposition chamber e.g.substrate heater assembly. Indeed, we have demonstrated that exposure ofa pre-deposited ZIS film to sulfur (at the substrate temperature equalto that used for ZIS deposition) decreases the dark conductivity andincreases the photoconductivity of the ZIS film. Data from thisexperiment is shown in Table 3 below. The increased photoconductivitycould be partly responsible for the gain in fill factor observed whensulfur is used.

Deposition of the buffer layer can be done as part of a continuousprocess, while the deposition system is maintained under vacuum betweendeposition of the CIGS and buffer deposition. However, if buffer is tobe applied to the CIGS layer from solutions, such as by application of athiourea solution (e.g., by dipping or spraying), the CIGS must first beremoved from the vacuum deposition apparatus.

TABLE 3 Dark and photo- currents for a ZIS film before and after sulfurtreatment (40 V applied, light one sun irradiance) Dark currentPhotocurrent Condition of ZIS film (nA) (nA) As-deposited (46 mg ZISevaporated at 0.07 0.12 T_(s) = 194° C.) After sulfur treatment (36 mg Sevaporated 0.02 0.72 at T_(s) = 194° C.)It was also found that a pre-treatment of the CIGS in asulfur-containing solution of thiourea (H₂NCSNH₂) was effective for mostsamples of CIGS in further improving the performance of CIGS solar cellswith ZIS buffer layers. In developing this treatment, the followingvariables were explored: solution molarity, solution temperature, andCIGS immersion time. Most effective was a 1M/85° C./30 min combination.Table 2 above shows the result obtained in run 140 using such atreatment applied before a standard ZIS deposition. The cell efficiencywas 8.5%. The benefit of the treatment relative to a simple water dip isevident from the results of run 181 (conducted on matching samples ofCIGS). In run 181A (water dip) the cell efficiency was 5.2%, whereas inrun 181B (thiourea treatment of the CIGS) the cell efficiency was 7.0%.

EXAMPLE

This example describes the fabrication of the solar cell reported in run51 of Table 1. The starting sample consists of a piece of glass bearinga sputtered Mo back contact and a film of CIGS of about 2.5 μm inthickness. The sample was rinsed in de-ionized water for one minute,dried in a stream of nitrogen, and mounted in a vacuum evaporator. Thesource crucible was made of graphite. 10 mg of ZnIn₂Se₄ (ZIS) was placedin the crucible, together with 19 mg of sulfur. The system was pumped toa pressure of 1.2×10⁻⁶ Torr using a diffusion pump, and the substratewas heated to 185° C. The crucible was heated using a resistive heatercausing the evaporation of the sulfur followed by the ZIS. The samplewas cooled to about 50° C. before removing it from the chamber. A singlelayer of transparent and conductive ZnO:Al was sputtered for 9 minutesonto the ZIS. The sample was removed from the sputtering chamber anddevices were defined by mechanical scribing. The devices (solar cells)were characterized by standard current-voltage and quantum efficiencymeasurements. The devices exhibited a conversion efficiency of 9.4% (seeTable 1 for details). A companion cell was prepared in identical fashionusing the same batch of CIGS but using ZIS only with no sulfur. Theefficiency of these devices was 7.0%, resulting from significantlyinferior fill factor and voltage, and slightly lower current (FIG. 3reveals the reduced short wavelength response).

1. A thin film photovoltaic device comprising: a) a substrate; b) afirst conductor thin film layer c) a chalcopyrite semiconductor thinfilm layer; d) a buffer layer consisting essentially of a defectchalcopyrite semiconductor with included sulfur; and e) a transparentconductor layer wherein the defect chalcopyrite semiconductor is formedprimarily of Group II, III, and VI elements.
 2. A device of claim 1 inwhich the chalcopyrite semiconductor is copper (indium, gallium)selenide.
 3. A device of claim 1 in which the buffer layer consistsessentially of ZnIn₂(Se_(1-x), S_(x))₄, where x is less thanapproximately 0.6.
 4. A device of claim 1 in which the transparentconductor layer consists essentially of a resistive layer of a metaloxide and a superimposed conductive layer of a doped metal oxide.
 5. Adevice of claim 1 in which the buffer layer is from 4 nm to 30 nm thick.6. A device of claim 1 in which the first conductor layer comprises Mo.7. A photovoltaic device consisting essentially of: a) a substrate; b) athin film layer of Mo; c) a thin film layer of copper (indium, gallium)selenide; d) a buffer layer consisting essentially of zinc indiumselenide, deposited in conjunction with provision of sulfur; and e) atleast one additional layer, comprising a doped transparent conductor. 8.A photovoltaic module comprising a plurality of monolithically connecteddevices of claim
 1. 9. A photovoltaic module comprising a plurality ofmonolithically connected devices of claim
 7. 10. A method for theproduction of a thin film photovoltaic device, comprising depositing thefollowing thin film layers on a substrate in a vacuum depositionapparatus: a) a conductor layer; b) a copper (indium, gallium) selenidelayer; c) a buffer layer; and d) a transparent conductor layer, whereinthe buffer layer consists essentially of ZnIn_(x)Se_(y)S_(z), andwherein x, y, and z are nonzero.
 11. A method of claim 10 in which theZnIn_(x)Se_(y)S_(z) is deposited from a vapor phase.
 12. A method ofclaim 11 in which the ZnIn_(x)Se_(y)S_(z) is deposited by vacuumevaporation during which the ZnIn_(x)Se_(y)S_(z) is maintained at avacuum evaporation temperature.
 13. A method of claim 12 in which theevaporation utilizes source materials consisting essentially of p gramsof ZnIn_(x)Se_(y)S_(z) and q grams of sulfur, where q=np and n is lessthan
 4. 14. A method of claim 13 in which n is from 0.3 to
 2. 15. Amethod of claim 13 in which the ZnIn_(x)Se_(y)S_(z) source material ispreheated to a pretreatment temperature significantly less than thevacuum evaporation temperature, for a time sufficient to drive offsuperficial selenium.
 16. A method of claim 15 in which the pretreatmenttemperature is from 250° C. to 600° C.
 17. A method of claim 13 in whichthe evaporation source materials are loaded as a single source and heldat progressively higher temperatures such that at least a portion of thesulfur is evaporated prior to evaporation of the ZnIn_(x)Se_(y).
 18. Amethod of claim 13 in which S and ZnIn_(x)Se_(y) source materials areloaded as separate sources and heated such that a predominant portion ofthe S is evaporated prior to evaporation of the ZnIn_(x)Se_(y).
 19. Amethod of claim 11 in which the vapor flux is restricted to approximateat least one point source.
 20. A method of claim 11 in which the vaporflux is restricted to approximate a line source.
 21. A method of claim11 in which the ZnIn_(x)Se_(y)S_(z) is deposited by sputtering from atarget.
 22. A method of claim 21 in which the target is a compoundtarget.
 23. A method of claim 21 in which the target is a Zn,Incombination.
 24. A method of claim 23 in which sulfur is supplied from agaseous source.
 25. A method of claim 24 in which the gaseous source isH₂S.
 26. A method of claim 11 in which the deposition source material isprepared by alloying the Zn and In to form an alloy and then reactingthe alloy with Se.
 27. A method of claim 11 in which the depositionsource material is prepared by simultaneously reacting the Zn, In, andSe.
 28. A method of claim 10 in which the substrate is maintained atfrom 100 to 350° C. during at least a portion of the buffer layerdeposition.
 29. A method of claim 10 in which the vacuum depositionapparatus is maintained under vacuum between deposition of the copper(indium, gallium) selenide layer and the buffer layer.
 30. A method ofclaim 10 in which deposition of the buffer layer is performed afterbreaking of the vacuum.
 31. A method of claim 10 in which the bufferlayer consists of a plurality of sublayers.
 32. A method for theproduction of a thin film photovoltaic device, comprising depositing thefollowing thin film layers on a substrate in a vacuum depositionapparatus: a) a conductor layer; b) a copper (indium, gallium) selenidelayer; c) a buffer layer deposited in conjunction with provision ofsulfur; and d) a transparent conductor layer, wherein the buffer layerconsists essentially of ZnIn_(x)Se_(y)S_(z), and wherein x and y arenonzero.